Taylor Hobson Limited, a division of Ametek Inc., manufactures a number of surface profile or form measuring metrological instruments which use a measurement probe to determine the profiles and geometries of surfaces. In many of these instruments, a mechanical stylus is employed and relative movement is effected between the stylus and the component along a measurement path so that surface changes in the z direction cause an arm carrying the stylus to pivot or rotate about an axis as the stylus follows these surface changes; rotation of the arm, and thereby z displacement, is then measured using an interferometric gauge.
One type of such a metrological instrument manufactured by Taylor Hobson is the Form Talysurf PGI® series. This series of instruments has a particularly good range-to-resolution ratio and so is capable of making measurements of both form and surface roughness or texture on surfaces having a significant degree of form. This makes the use of the Form Talysurf PGI series of instruments particularly advantageous for the measurement of aspheric diffractive lenses (and moulds therefor) as may be employed in infra red imaging devices, digital cameras, mobile telephones, and optical storage devices such as Digital Versatile Disc (DVD) recorders and players.
The form of an aspheric diffractive lens can be described as an aspheric lens having a diffractive structure superposed thereon. The combination in a single lens of both aspheric and diffractive components enables correction in the same lens of both chromatic aberrations and field dependent aberrations. Typically, the diffractive form of such lenses can be mathematically decomposed into a component having a series of steps that are concentric with the axis of rotation of the aspheric component (the aspheric axis) and a component having a rotational polynomial term that ensures that the diffractive structure remains within a specified uniform height band. As another possibility, the diffractive structure may be arranged to have a constant radial distance between the steps.